Harvard Psychological Studies, Volume 1 eBook

However, before I began this second series, in which
one of the chief variations was to be in the weights
of the different points, I made a brief preliminary
series of experiments to determine in a general way
the influence of pressure on judgments of point distances.
Only three distances were employed, four, six and
twelve centimeters, and three weights, twelve, twenty
and forty grams. Table III. shows that, for three
men who were to serve as subjects in the main experiments
that are to follow, an increase in the weight of the
points was almost always accompanied by an increase
in the apparent distance.

In the standard distances the points
were each weighted to 6 grams. The first
three figures signify that a two-point distance
of 4 cm., each point weighing 6 grams, was judged
equal to 3.9 cm. when each point weighed 12 grams.
3.2 cm. when each point weighed 20 grams, etc.
Each figure is the average of five judgments.

Now the application of this principle in my criticism
of Parrish’s experiments, and as anticipating
the direction which the following experiments will
take, is this: if we take a block such as Parrish
used, with only two points in it, and weight it with
forty grams in applying it to the skin, it is plain
that each point will receive one half of the whole
pressure, or twenty grams. But if we put a pressure
of forty grams upon a block of eight points, each point
will receive only one eighth of the forty, or five
grams. Thus, in the case of the filled space,
the end points, which play the most important part
in the judgment of the distance, have each only five
grams’ pressure, while the points in the open
space have each twenty grams. We should, therefore,
naturally expect that the open space would be overestimated,
because of the decided increase of pressure at these
significant points. Parrish should have subjected
the blocks, not to the same pressure, but to a pressure
proportional to the number of points in each block.
With my apparatus, I was easily able to prove the
correctness of my position here. It will be seen
in Tables IV. to VIII. that, when the sum of the weights
of the two end points in the open space was only just
equal to the sum of the weights of all the points
in the filled space, the filled space was underestimated
just as Parrish has reported. But when the points
were all of the same weight, both in the filled and
the open space, the filled space was judged longer
in all but the very short distances. For this
latter exception I shall offer an explanation presently.